1. Field of the Invention
The present invention relates to image processing. More to particularly, the present invention relates to a novel and improved image signal compression scheme utilizing adaptively sized blocks and sub-blocks of encoded discrete cosine transform (DCT) coefficient data.
2. Description of the Related Art
In the field transmission and reception of television signals, various improvements are being made to the NTSC (National Television Systems Committee) System. Developments in the field of television are commonly directed towards a high definition television (HDTV) System. In the development of HDTV, system developers have merely applied the Nyquist sampling theorem and low pass filtering design with varying degrees of success. Modulation in these systems amounts to nothing more than a simple mapping of an analog quantity to a value of signal amplitude or frequency.
It has most recently been recognized that it is possible to achieve further improvements in HDTV systems by using digital techniques. Many of the proposed HDTV transmission formats share common factors. These systems all involve digital processing of the video signal, which necessitates analog-to-digital (A/D) conversion of the video signal. An analog transmission format is then used thereby necessitating conversion of the digitally processed picture back to analog form for transmission.
The receiver/processor must then reverse the process in order to provide image display. The received analog signal is therefor digitized, stored, processed and reconstructed into a signal according to the interface format used between the receiver/processor and the HDTV display. Furthermore the signal is most likely converted back to analog form once more for display. It is noted however that the proposed HDTV formats utilize digital transmission for transmission of control, audio and authorization signals.
Many of the conversion operations mentioned above, however, may be avoided using a digital transmission format which transmits the processed picture, along with control, audio and authorization signals, using digital modulation techniques. The receiver may then be configured as a digital modem with digital outputs to the video processor function. Of course, the modem requires an A/D function as part of operation, but this implementation may only require a 4-bit resolution device rather than the 8-bit resolution device required by analog format receivers.
Digital transmission is superior to analog transmission in many ways. Digital transmissions provide efficient use of power which is particularly important to satellite transmission and in military applications. Digital transmissions also provides a robustness of the communications link to impairments such as multipath and jamming. Furthermore digital transmission facilitates ease in signal encryption, necessary for military and many broadcast applications.
Digital transmission formats have been avoided in previous HDTV system proposals primarily because of the incorrect belief that they inherently require excessive bandwidth. Therefore in order to realize the benefits of digital transmission, it is necessary to substantially compress the HDTV signal. HDTV signal compression must therefor be achieved to a level that enables transmission at bandwidths comparable to that required by analog transmission formats. Such levels of signal compression coupled with digital transmission of the signal will enable a HDTV system to operate on less power with greater immunity to channel impairments.
It is therefore an object of the present invention to provide a novel and improved method and system for compressing HDTV signals that will enable digital transmission at bandwidths comparable to that of analog transmissions of convention TV signals.